1. Field of the Invention
This invention generally relates to a magnetic lens which may be configured to apply a magnetic field to a charged particle beam, and more particularly, to a sectored magnetic lens and a control apparatus for a magnetic lens which may be incorporated into a scanning electron microscope system.
2. Description of the Related Art
As the dimensions of semiconductor devices continue to shrink with advances in semiconductor materials and processes, the ability to examine microscopic features and to detect microscopic defects has become increasingly important in the successful fabrication of advanced semiconductor devices. Significant research continues to focus on increasing the resolution limit of metrology tools that are used to examine microscopic features and defects. Optical microscopes generally have an inherent resolution limit of approximately 200 nm and have limited usefulness in current manufacturing processes. Microscopes that utilize electron beams to examine devices, however, may be used to investigate feature sizes as small as, e.g., a few nanometers. Therefore, tools that utilize electron beams to inspect semiconductor devices are increasingly becoming integral to semiconductor fabrication processes. For example, in recent years, scanning electron microscopy has become increasingly popular for the inspection of semiconductor devices. Scanning electron microscopy generally involves scanning an electron beam over a specimen and creating an image of the specimen by detecting electrons that are reflected, scattered, and/or transmitted by the specimen.
The electron optical system of a scanning electron microscope generally includes an electron source, a device or a plurality of devices configured to focus an electron beam generated by an electron source, a detector or a plurality of detectors configured to detect electrons reflected, scattered, or transmitted by the specimen, and a control system. A thermal field emission source may typically be used as an electron source, and the energy of the electron source may be controlled by an emission control electrode and an anode. The electron beam may pass through a magnetic condenser lens configured to collimate the electron beam. An initial deflection system may also be located near the electron source. An initial deflection system may be configured to correct alignment, stigmation and blanking of the beam. Prior to passing through a magnetic objective lens, the beam may also be passed through a beam limiting aperture and one or more electrostatic pre-lens deflectors. The magnetic objective lens may further focus the electron beam to a spot size of, for example, approximately five nanometers. As used herein, the term “spot size” is generally defined as a lateral dimension of an electron beam incident upon a specimen. A magnetic objective lens may typically include a lower pole piece, an intermediate electrode, and an upper pole piece.
An electron beam exiting a magnetic objective lens may be scanned across a specimen. Typically, the electron beam may be scanned in a first direction while the stage supporting the specimen may be moved in a direction perpendicular to the first direction. A plurality of detection systems may be used to detect secondary electrons, back-scattered electrons, and transmitted electrons that may be produced when the electrons contact the specimen. Examples of scanning electron microscope systems are illustrated, for example, in U.S. Pat. No. 4,928,010 to Saito et al., U.S. Pat. No. 5,241,176 to Yonezawa, U.S. Pat. No.5,502,306 to Meisburger et al., U.S. Pat. No. 5,578,821 to Meisburger et al., U.S. Pat. No. 5,665,968 Meisburger et al., U.S. Pat. No. 5,717,204 to Meisburger et al., U.S. Pat. No. 5,869,833 to Richardson et al., U.S. Pat. No. 5,872,358 to Todokora et al., and U.S. Pat. No. 5,973,323 to Adler et al., and are incorporated by reference as if fully set forth herein.
The performance of a scanning electron microscope may vary depending on, for example, the capability to focus an electron beam on a small target area. High voltage electrons may penetrate deep into a semiconductor substrate or a portion of a semiconductor formed upon a semiconductor substrate thereby damaging the substrate or the device and rendering it unsuitable as a working device such as an integrated circuit. Therefore, low voltage electron beams may typically used to analyze delicate semiconductor specimens that otherwise might be damaged by high voltage electron sources. The primary factor that reduces resolution in the low acceleration voltage region is blur of the electron beam due to chromatic aberration. Dispersion in the energy of the electron beam emitted from the electron source typically causes chromatic aberration. As such, significant effort has been focused on improving the performance of a scanning electron microscope by enhancing the ability of the magnetic objective lens to reduce chromatic aberrations in an electron beam source especially in low voltage particle beams.
Traditionally, magnetic lenses may be axially symmetric and may produce axially symmetric magnetic potentials and magnetic fields. An example of such a magnetic lens is illustrated, for example, in U.S. Pat. No. 6,002,135 to Veneklasen et al. and is incorporated by reference as if fully set forth herein. A magnetic lens may include an inner pole piece that may have a cylindrical upper portion and a conical lower portion that may be substantially enclosed by an outer pole piece. The outer pole piece may also have a cylindrical upper portion and a conical lower portion corresponding to the inner pole pieces. A solenoidal excitation coil may be disposed between the inner pole piece and the outer pole piece. When a current is applied to the excitation coil, an axial focusing field may be generated within the lens by magnetic flux from the inner and outer pole pieces. The axial focusing field may be used to focus an electron beam. Shielding rings may be arranged between the upper and lower portions of the inner pole piece to reduce the air gap between the pole portions. The shielding rings may also provide a return path for deflection flux that may otherwise radiate through the gap and induce eddy currents in outer pole pieces and excitation coil. Deflection coils may also be included within the lens along the beam path.
Variable axis lenses have also been developed to focus electron beams. Variable axis lenses incorporate supplementary lenses or supplementary deflectors in the magnetic lens to provide some correction of electron beam paths that may be laterally displaced from an optical axis of the lens. The supplementary lenses and deflectors may be energized based on the lateral displacement of the beam path. Although electron beams may be deflected by this lens, astigmation may still be a problem. Therefore, a separate astigmation compensator may also be included in such a lens. Alternatively, an astigmatism-correction deflector system may be arranged within a variable axis lens adjacent the internal surface of the supplementary deflectors. Such deflectors may be constructed of an octapole three-stage coil in which each octapole includes two tetrapole sets. A deflection field coil may be added to one of the tetrapole coil sets of the octapole. An example of a variable axis lens is illustrated in U.S. Pat. No. 5,952,667 to Shimizu and is incorporated by reference as if fully set forth herein. The incorporation of a separate astigmator octapole forces the beam to pass through the center of this octapole. The overall alignment of the lens system, however, may be non-colinear due to the incorporation of such a separate feature. Therefore, complexity of the overall alignment of the system increases when the charged particle beam is forced to pass through successive non-colinear points.
There are, however, several disadvantages to the lens systems described above. For example, axially symmetric lenses may typically suffer from hysteresis, large inductance of the excitation coil, and thermal stability problems. Hysteresis may cause a relationship between the excitation coil current and the deflected beam position to depend upon past deflection history. Therefore, accurate focus of an electron beam using the magnetic lens may be extremely difficult to maintain and control. Additionally, large inductance of the excitation coil may cause thermal stability problems due to heat generated by the lens or the excitation coils. Therefore, the center of the lens may shift due to thermal expansion of the materials used to construct the lens.
Immersion lenses are also limited in their application to a variety of specimens. Immersion lenses are generally designed to limit aberrations of an electron beam by reducing the distance between the specimen and the maximum magnetic field. The distance between the specimen and the maximum magnetic field may be reduced by placing the specimen near or within the lens. Examples of immersion lenses are illustrated in U.S. Pat. No. 5,089,428 to Da Lin et al. and is incorporated by reference as if fully set forth herein. Due to space limitations, immersion lenses may not be able to accommodate a large specimen such as a semiconductor substrate. For example, 200 mm wafers, or semiconductor substrates, are already being used in the development and production of semiconductor devices. Efforts are also underway to further increase the size of semiconductor substrates to 300 mm. Modifying these lenses in order to accommodate such large semiconductor substrates may also adversely affect the performance of immersion lenses. Alternatively, reducing the size of the semiconductor substrate by cross-sectioning the wafer is not usually an option due to the cost associated with destroying a product wafer.
An asymmetric immersion lens may be configured to reduce the distance between a specimen and the strongest magnetic field of the lens. An asymmetrical lens, however, may be configured to produce a magnetic field that rises sharply just in front of a conical pole piece near the bore of the lens or the position at which the electron beam exits the magnetic lens. The magnetic field falls slowly toward a second pole piece or a magnetic housing. The specimen and the conical pole piece are disposed within the magnetic housing such that the specimen may be placed near the conical pole piece. Asymmetric immersion lenses may be more flexible to accommodate large specimen such as semiconductor substrates, but these lenses may have a reduced capability to detect secondary electrons. For example, because secondary electrons may be emitted at a point beyond the magnetic field peak, low energy secondary electrons may not be able to surmount the magnetic field maximum. In order to overcome low detection of secondary electrons, a conducting grid system may be included in the lens. The conducting grid system may include an auxiliary grid to accelerate the secondary electrons away from the inner surface of the lower pole piece, an extraction grid to reduce the axial velocity of the secondary electrons, and a restrain grid to turn back any uncollected secondary electrons. Therefore, in order to overcome the low detection of secondary electrons, a conducting grid system may be included in the lens.
In addition to the above disadvantages, the performance of magnetic lenses may also be limited due to changes in the magnetic field strength due to low frequency noise, drift in the performance of current drive electronics, drift due to eddy currents or superimposed fields from other sources, and drift in the magnetic field strength over time from other causes. Although a magnetic lens design may minimize these effects on the performance of the lens, it may not be possible to substantially eliminate magnetic field drift of the lens. For example, eddy currents due to magnetic flux leakage from a lens through a gap in the magnetic lens may adversely affect the performance of the magnetic lens. Because a magnetic lens must be designed with a bore to allow the electron beam to travel through the lens, however, it is impossible to seal the magnetic lens off completely. As a result, some of the magnetic field will inherently “leak” out of a magnetic lens. Therefore, the effects of eddy currents on the performance of a magnetic lens may not be completely eliminated due to usage requirements. Drift in the magnetic field may cause the electron beam to drift out of focus. Therefore, the overall resolution of a scanning electron microscope may also be reduced by the presence of the above sources of magnetic field drift. The functioning of the scanning electron microscope may, however, be dramatically improved by an accurate control system for the magnetic objective lens.
A control mechanism for a magnetic lens may generally include a device for sensing the current density of the electron beam at a position spaced from an axis along which an electron beam travels. For example, an alignment yoke disposed along the axis of the electron beam may receive a signal from the current sensing arrangement. Therefore, the alignment yoke may be mechanically shifted incrementally and orthogonally until a maximum current may be produced at the reference location. The beam may be centered due to the altered position of the alignment yoke thereby reducing aberrations in the projections lens. An example of such a focusing system is illustrated in U.S. Pat. No. 4,423,305 to Pfeifer and is incorporated by reference as if fully set forth herein. Mechanical focus methods, however, may be slow due to the time required for moving the devices. In addition, microscopic vibrations due to mechanical motion of the alignment yoke may need to settle before the magnetic lens may perform adequately. Therefore, mechanical focus methods may require additional time such that microscopic vibrations will not affect the performance of the magnetic lens.
An alternative control mechanism for a magnetic lens involves focusing and controlling an electron beam by determining the focal length at which a sample will be brought into focus. Focal length may be a function of the electron beam energy and the magnetic field strength. In this manner, one available control mechanism involves using an electron trajectory tracing program to measure the converging point, or focal length, for an electron beam by using measurements of the electron beam energy and the magnetic field strength. The magnetic field strength may be estimated by measuring a current in the lens coil. An adjustment to the current in the lens coil may be made to correct the converging point of the electron beam. Instantaneous magnetic field strength, however, may be determined by the present current and the history of all other currents in the coil which is commonly referred to as hysteresis. Hysteresis in the magnetic field strength may also be induced from frequent changes in the voltage supplied to a magnetic lens. For example, the lens current in a scanning electron microscope may often be automatically adjusted to a nominal beam voltage. As new specimens are being observed, the user may usually alter the beam voltage to bring the specimen into focus. Therefore, frequently adjusting the beam voltage may increase the complexity of a current history of the lens. As the complexity of the current history increases, hysteresis will also become more problematic. Therefore, estimating the magnetic field strength using measurements of the current in the lens coil may induce error in this method. In addition, the coil and the core materials may react in a non-ideal way to the frequent changes in the current being supplied to the coil. Therefore, additional electrical and magnetic characterization of the coil and the core materials may be necessary. A thorough degaussing procedure may reduce the effects of hysteresis, however, magnetic hysteresis typically remains a problem in most magnetic lenses.
Additional methods to control the electron beam focus have attempted to reduce the effects of hysteresis of the magnetic lens by keeping the lens current constant after an initial manual focus and calibration. An example of such a magnetic lens control method is illustrated in U.S. Pat. No. 4,999,496 to Shaw et al. and is incorporated by reference as if fully set forth herein. The control method involves varying the electron beam energy to alter the focus of the magnetic lens, as working distance changes such as when different areas of a specimen are viewed. In order to offset the effects of a new beam energy, the current in the scanning coils may be altered to maintain accurate magnification. Although such a method for focusing the electron beam may reduce deleterious effects of hysteresis in the current of the magnetic objective lens, other factors that may lead to defocusing may not be addressed in this design. For example, as mentioned above, other factors that may hinder the performance of a magnetic lens may include thermal changes in the material properties, drift in the current drive electronics, drift in the magnetic field due to eddy currents, and drift due to superimposed fields from other sources.
Furthermore, in many scanning electron microscope systems, coarse and fine focusing of the magnetic lens may typically be performed manually by an operator. The operator may alter the focus of the magnetic lens by controlling the electric current of the magnetic lens. In order to obtain the desired magnetic field strength, the operator may alter the current while observing the effects of the magnetic field on an image of a specimen until the optimal performance is achieved. The image may be observed using a display system such as a color or grayscale monitor. The resulting magnetic field, however, may follow a nonlinear relationship with the current due to hysteresis in the magnetic lens and the behavior of the coil and the core materials in response to the change in current. This iterative method also depends on operator judgement and is consequently subject to error. More importantly, the additional sources of magnetic field drift described above may also cause this magnetic field to be irreproducible.
Accordingly, it would be advantageous to improve the performance of magnetic lens and a magnetic lens control apparatus that may be used to focus an electron beam by reducing the effects of, for example, hysteresis and thermal instability in the magnetic field strength of a magnetic lens.